1. Field of the Invention
The present invention relates to an apparatus for measuring the remaining amount of bobbin thread wound on a bobbin accommodated in a shuttle of a sewing machine.
2. Related Art Statement
Japanese Non-Examined Utility Model Application laid open under Publication (JITSU-KAI-SHO) No. 61(1986)-180685 discloses an example of the remaining bobbin-thread amount measuring apparatus. The disclosed apparatus is of optical type wherein, when the rotation of a rotating hook of a shuttle is stopped, a light is emitted toward the outer surface of a roll of bobbin thread wound on a bobbin accommodated in the rotating hook and the light reflected from the roll of bobbin thread and detected by an optical sensor is utilized to determine the remaining amount of the thread left on the bobbin. However, the sensitivity of the optical sensor is lowered due to dust or oil adhered thereto. Another problem with the measuring apparatus is that the sensitivity of the optical sensor changes depending upon colors of bobbin thread. Thus, in some cases, the optical-type apparatus cannot measure the remaining amount of bobbin thread with sufficiently high accuracy.
Meanwhile, Japanese Examined Patent Application laid open for opposition purpose under Publication (TOKU-KO-SHO) No. 61(1986)-43075 discloses a near-end detecting apparatus. The disclosed apparatus is of mechanical type which includes (a) a measurement bar which is displaceable along a displacement path from outside a shuttle toward the axis line of a bobbin accommodated in the shuttle, (b) a measurement-bar driver which displaces the measurement bar along the displacement path so that a free end of the bar contacts the outer surface of a roll of bobbin thread wound on the bobbin, after the rotation of the shuttle has been stopped, the driver retracting the measurement bar away from the outer limit of a locus of rotation of the shuttle before the rotation of the shuttle is started, and (c) a near-end microswitch which is adapted to close when the displacement of the measurement bar results in the contact thereof with the bobbin thread indicating that the displacement exceeds a predetermined value during the time that the bobbin has been gradually consumed, the closing of the microswitch triggering an alarm.
Thus, the near-end detecting apparatus identifies that the bobbin thread wound around the bobbin is reduced to near-end, i.e., near-zero. However, the apparatus cannot measure a remaining amount of the bobbin thread left on the bobbin.
In the event that the location of disposition of the near-end microswitch is changed even a slight distance, the microswitch triggers an alarm at an inaccurate displacement amount of the measurement bar different from the above-indicated predetermined value. Therefore, the near-end detection apparatus suffers from insufficient accuracy of the detection of near-end.
Furthermore, the near-end detecting apparatus has a third problem that the measurement bar may jam up by being interfered with by other parts when the bar is displaced out of the shuttle after the free end of the bar has contacted the roll of bobbin thread on the bobbin. In that event, the measurement bar cannot be retracted away from the outer limit of the locus of rotation of the rotating hook and, if the rotation of the hook is started, then the hook collides with the measurement bar so that both the hook and the bar are damaged.
For solving the above-indicated third problem of the near-end detecting apparatus, Japanese Non-Examined Utility Model Application laid open under Publication (JITSU-KAI-SHO) No. 63(1988)-136591 discloses an improved near-end detecting apparatus. The improved, third apparatus includes a first sensor which identifies that a measurement bar has been retracted away from the outer limit of rotation locus of a shuttle, in addition to a second sensor which detects the near-end of bobbin thread, i.e., identifies that the displacement of the measurement bar till the contact thereof with the bobbin thread left on a bobbin has exceeded a predetermined value corresponding to the near-end of bobbin thread.
However, the third apparatus suffers from the complicated construction due to the employment of the two sensors, and accordingly it suffers from increased cost of manufacture.